MICRO-AND NANOENGINEERING APPROACHES TO DEVELOPING GRADIENT BIOMATERIALS SUITABLE FOR INTERFACE TISSUE ENGINEERING - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 67

of the technique is on the order of a few micrometers. 109 However, optimization of
the stamp building technique allows for a resolution under 100 nm. 110 Although the
m

CP technique is simple to use and has several merits, it also has a few drawbacks.
For example, stamp swelling during the inking process can result in larger imprinted
patterns or resolution problems caused by overdiffusion of the ink; in addition, stamp
deformation, such as pairing, buckling, or roof collapse, during contact with the
substrate surface results in distorted patterns. m CP has been widely used to modify
surfaces, and gradients were built by applying different pressures on the stamp,
varying the contact time between the stamp and the substrate, using a nonplanar
stamp, gradually soaking the stamp in ink, or gradually depositing the ink on the
substrate. 111 Von Philipsborn et al. proposed a protocol to print discontinuous
gradients of axon-guidance proteins by a lift-off method or by a casting method. 112
After overnight culture with embryonic chick retinal ganglion cell axons, protein
patterns were analyzed for their interactions with axons by fluorescent labeling.
Thus, CP is an important technique that can be used to fabricate bioengineered

systems and devices.

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3.3.9 Electrospinning

Electrospinning is an easy and versatile technique based on the ejection of a
polymeric jet from the tip of an electrically charged syringe, the spinneret, followed
by its collection onto a counter electrode, resulting in the formation of fibers with
sizes usually ranging from 10 nm to a few micrometers. By manipulating the
electrospinning process, the thickness and orientation (aligned or random) of
nanofibers can be controlled to match the structure of the targeted tissue. 113
Electrospun fibers have adequate mechanical properties, high porosity, and a large
surface-to-volume ratio, which are beneficial properties for interactions with cells
and for tissue engineering applications. 114,115 Thus, nanofiber scaffolds have been
widely investigated for ligament, 116 meniscus, 117 and bone tissue engineering. 118,119
Nanofiber surfaces can also be modified by bioactive molecules to increase their
cellular compatibility. 120,121 One approach to generating a gradient is based on the
surface modification of the ejected fibers after electrospinning. Shi et al. incorporated
a fibronectin concentration gradient in a polymethylglutarimide (PMGI) electrospun
mesh scaffold by wetting the scaffold at a controlled speed from bottom to top with
fibronectin solution in a vertical PDMS microchamber. 122,123 Loaded with NIH3T3
cells and cultured for 24 h, the scaffold showed a cell density gradient decreasing
from 1400 cells/mm 2 (at the bottom of the scaffold) to 100 cells/mm 2 (at the top of
the scaffold), following the fibronectin concentration gradient. To mimic the tendon-
bone interface, Li et al. coated the electrospun nanofibers of PLGA and PCL mats
with a calcium phosphate mineralization gradient by wetting the scaffolds at a
controlled speed from bottom to top with 10-fold concentrated simulated body fluid
(10 SBF) solution in a glass. 7 The mineralization gradient decreased from 37.8% (at
the scaffold bottom) to 0.7% (at the scaffold top) in calcium phosphate and from
33.9% to 0.8% for the PLGA and PCL scaffolds, respectively. When the PLGA
scaffold is subjected to uniaxial tensile deformation, the Young's modulus follows